Conventional thermal conductivity detectors used in detecting sample gas normally employ two sensors in a bridge configuration. Both sensors are enclosed in a constant temperature oven. The sensors are of the type, e.g., thermistors or hot wires, whose resistances vary as a function of their temperatures. The two sensors ideally are matched identically at the factory to have identical electrical characteristic.
These detectors operate in the following manner: Carrier gas (e.g., helium, argon, etc.) flows through individual cells containing the thermistor (or hot wires) at some constant rate. A bridge balance is established under these conditions. Next, the sample to be analyzed is introduced into the carrier gas flowing through one thermistor identified as the "sample" or "sensor" side of the bridge. No sample is introduced into the carrier gas flow through the other thermistor identified as the "reference" side of the bridge. The sample is in gaseous form, usually the evolved product(s) of combustion of a solid sample. The mixture of the sample gas (or gases) has a thermal conductivity higher (or lower) than the carrier. This change in thermal conductivity of the gas mixture passing over the "sample" thermistor causes the thermistor temperature to change by virtue of heat loss (or gain), thereby changing its resistance. Since the "reference" thermistor resistance remains constant, the bridge becomes unbalanced, causing a voltage signal to be generated. This signal may be amplified by appropriate electronic circuitry. Finally, samples whose constituents and their weight percentages are known are used to calibrate the apparatus.
Matching sensors to have identical electrical characteristic is costly and almost never perfectly achievable. Since these sensors must be balanced with respect to several parameters, including ohmic resistance at room and operating temperatures, offset, i.e., differences in resistance at operating temperatures and drift, i.e., variation of resistance with time, the testing required to insure the required balance imposes significant costs, particularly in those cases such as drift, where the defect cannot be detected until the detector is completely assembled.
The foregoing described problems can be virtually eliminated by using only one sensor and substituting a variable resistance for the previously required sensor.
In carrying out the present invention, an arm of a Wheatstone bridge circuit carrying the sensor is the only part of the bridge circuit enclosed in a constant temperature oven. The reference arm, i.e., the one containing the variable resistance, is located outside the oven. In operation, a carrier gas is caused to flow over the sensor in the oven. The variable resistance is changed until the voltage output from the bridge is zero or some other acceptable value. Then, a sample gas is introduced and intermingled with the carrier gas. The change in thermal conductivity of the mixture from the carrier gas is representative of the sample. This unbalances the bridge so that the bridge output signal is indicative of the sample.